1. Field of the Invention
The present invention relates to a knock detecting apparatus for use in an ignition timing control system, etc., for internal combustion engines, which control system has a function to adjust the ignition timing to a predetermined knock level by detecting the knock from vibrations produced outside of the cylinder due to the pressure in the cylinder of the internal combustion engine.
2. Description of the Prior Art
It is generally known tht there is a close correlation between the ignition timing and the pressure in the cylinder. In this regard, the cylinder pressure caused by the explosion of a mixture, when no knocking is produced, does not involve higher harmonics (which usually include frequency components of 5 KHz-10 KHz or 11-13 KHz and which are related to the bore diameter of the engine cylinder and generated by intermittent, rapid combustions), however, when knocking tends to occur higher harmonics begin to appear in the vicinity of a maximum value of the cylinder pressure, and by this effect a vibration or sound is generated outside of the cylinder. According to close observation of the internal pressure signals generated in the cylinder or the generation of vibration or sounds outside of the cylinder, it is revealed that at the beginning of the knock occurence (the trace knock) higher harmonics begin to appear at an engine crank angle at which the engine pressure reaches a maximum value, and when the intensity of the knocking increases gradually (from light knock to heavy knock), the higher harmonics appear to a great extent at the advance side (i.e. ignition side) of the maximum value of the internal pressure. Thus, the engine efficiency will be improved remarkably if generation of the vibration or sounds generated outside of the cylinder due to the knocking is detected accurately, and if the ignition timing is controlled by feeding back the detected data. However, the actual situation is that no knock detecting apparatus is available which is capable of detecting the knocking conditions precisely enough to provide the data to be fed back, and which can operate stably under severe surrounding conditions required for use with vehicles.
Generally, there are two types of detectors for such detections as mentioned above. One type utilizes a piezoelectric type acceleration detector which is generally used for detecting vibration, and the frequency characteristic of this detector is higher than the knocking frequency and this characteristic becomes flat below the resonance point (hereinafter referred to as a non-resonance type), and the other is a resonance type which is under study by the inventors of the present application and which exhibits resonance characteristic at the knocking frequency. As the resonance point in the non-resonance type is higher than the knocking frequency, the sensitivity in a low frequency band including the knocking frequency below the resonance point is almost constant. Accordingly, it is possible in principle to detect the knocking over the whole range of the knocking frequency. However, during the engine operation, vibration noise such as caused by a valve seating vibration occurs frequently, and the signal-to-noise (S/N) ratio of the knock signal to the vibration noise will be deteriorated. For this reason it is practically impossible to detect the knocking by the non-resonance type detector during high speed engine operation. Also it is difficult to detect slight knocking even during low speed operation since the overall detection sensitivity is low.
In the resonance type, the detection sensitivity is improved remarkably at a specific frequency near the resonance point, and since vibration noise having other frequencies is hard to detect, the S/N ratio at the knocking frequency and the sensitivity are improved remarkably.
However, the resonance type has also the drawback in that when the degree of resonance (the resonance sharpness Q) becomes high, the detection frequency range is necessarily narrowed. Hence, the higher the Q, the more difficult the knock detection becomes due to a shift of the resonance frequency, or due to a slight shift of the knocking frequency which changes depending on the state of combustion.
Eventually, it is necessary for an ideal knock detecting apparatus to have the S/N ratio and sensitivity comparable to that of the resonance type, over a wide range within the whole frequency band of knocking.